The present invention relates to a method for storing and mixing two co-reactive basic materials for dental amalgam, usually a silver alloy (in a typical case containing--apart from silver--also tin, copper and zinc), known in the art as alloy, functioning as one component and pure mercury as the second component.
The invention also relates to a capsule for storing and mixing the two co-reactive components by employing the method in accordance with the invention, and furthermore relates to a method in manufacturing this capsule.
In the sphere of dentistry many proposals have been made for capsules for amalgam whereby quantities proportioned in advance of the two co-reactive components are stored in the capsule separate from each other and can be brought together and mixed directly in the capsule so as to react with each other whilst forming amalgam directly prior to its proposed use.
The capsules now available on the market include a chamber for each component with an openable point of separation between the chambers. Several different principles are employed to open the point of separation.
In one embodiment (U.S. Pat. No. 3,275,302) a ball or disc which rests on a seating between the chambers forms the point of separation between the chambers, the said separation being opened by shaking the capsule so as to mix the components with each other. The ball or disc at the same time forms a mixing body, known as a pistil, when the capsule is shaken during mixing. One significant disadvantage of this capsule consists in the fact that the capsule is sensitive as regards its position; it must not be turned upside down before mixing is to take place.
In another embodiment (U.S. Pat. No. 3,357,545) the capsule contains two portions which are adjustable relative to each other with an aperture between the chambers formed by the sections which can be opened and closed by turning or moving the sections relative to each other.
The method is also known (U.S. Pat. No. 3,625,349) wherein, in a capsule which consists of two rotatable components arranged excentrically relative to each other, mercury is provided in a small bag of plastic foil which is torn open by rotating the sections relative to each other so that the mercury can be pressed out of the bag through an aperture between the sections and into a chamber which contains the alloy.
Capsules having two sections which are telescopically displaceable mutually are also known wherein the sections are pressed telescopically together so that a membrane between the chambers which functions as the point of separation will by this means be broken, either because one section is pressed directly against the membrane (U.S. Pat. No. 3,451,540) or by one section pressing on the mercury which in turn presses against membrane (U.S. Pat. No. 3,595,439). The membrane can also be shaped like a small bag of plastic foil which contains the mercury and which rests loosely in a chamber which contains the alloy. The plastic bag is broken up in that the capsule sections are pressed telescopically together when the components are to be mixed with each other.
Apart from the fact that these known dental capsules are complicated in design, in certain cases extremely complicated, and hence relatively expensive to produce which is a distinct disadvantage, when--as here--they are to be throwaway capsules they suffer from the disadvantage that a particular manipulation of the capsule is required before the components can mix, and furthermore this is a manipulation which in some cases requires the exercise of quite a great deal of force. If the mercury is located in a small plastic bag inside the chamber for the alloy, there is furthermore the shortcoming that the plastic foil remains behind in the resultant amalgam in the form of one or more pieces and has to be separated out from the amalgam when this is removed from the capsule for use.
A dental capsule has been proposed which does not require any special manipulation before the mixing of the components (Swedish patent application No. 7906465-5). The capsule comprises a chamber for each of the components, and the chamber which contains the alloy also contains a rammer which when the capsule is shaken knocks repeatedly against a displaceable separating wall between the chambers. By this means the separating wall is pressed successively into the chamber containing the mercury, thus opening up passages which permit the mercury, during the movement of the separating wall, to be forced past the said wall into the chamber with the alloy. However this capsule is just about as complicated as the capsules described above and furthermore requires fairly high precision during manufacture.
One problem which has either not been considered at all or only inadequately in the case of the dental capsules described above is that mercury in extremely fine-particle form can penetrate outwards between the displaceable sections which are rotatable or telescopically displaceable relative to each other, or in the joints between the sections of the capsule which have to be separated in order to take out the finished amalgam. When mixing of the components takes place whilst the capsule is being shaken at high frequency, which is performed in special shaking devices available for the purpose, the mercury is flung out in the form of microscopically small droplets which have a very large cumulative surface and thereby a correspondingly high vaporisation rate. It is known that the daily respiration of air contaminated by mercury vapour can result in chronic mercury poisoning, and consequently the fact is not unknown that in dental surgeries there is some risk of such poisoning, even though the frequency of its occurrence is assumed to be low. This occupational hygiene health risk is hence probably slight, but even so having regard to its consequences, it is well worth taking into account when designing dental capsules particularly since mercury can cause damage also in other ways, namely by giving rise to allergic reactions in certain persons.
The problem of mercury leakage from dental capsules during shaking of the capsule in order to prepare the amalgam has been discussed in the literature and has also been subject to investigations and proposals have also emerged for a dental capsule which is designed to eliminate the risk of mercury escape. This dental capsule (German laid-open specification No. 28 31 005.6) is relatively simple in design. One chamber has a welded-on or glued-on cover with a breakage point, and in this chamber which contains the alloy, the other chamber is located in the form of a bag of thin plastic foil which encloses the mercury. When the capsule is shaken the bag is broken open against the end walls of the capsules and when the amalgam is ready the cover is broken away from the rest of the capsule at the breakage point using a special puncturing device. Apart from the fact that in this case too there is the disadvantage of encountering foreign substances in the amalgam, i.e. the disintegrated plastic bag which can be in the form of one or more pieces, the attachment of the cover whilst providing a breaking point after the capsule has been filled with the components does involve some complication, as does the breaking up of the cover which cannot be performed just as comfortably as when separating two components which are pushed together.